This invention relates to improvements in downdraft gasifiers and to improvements in burners that heat external combustion engines using gas or solid particulate fuel. The invention more specifically relates to power generation equipment that combines a mechanical power source in the form of an external combustion engine, such as a free-piston Stirling engine, with a downdraft gasifier that supplies fuel gas from multiple forms of biomass to a burner for powering an external combustion engine.
There has long been a need for relatively small power generation equipment that can generate power for operating mechanical devices, such as water pumps or electrical devices like lighting and electrical motors, using locally available fuels as energy sources. One of the most promising options is an external combustion engine which is heated by fuel gas produced from biomass, such as dried manure, or dried vegetation in the form of sawdust, rice husks, corn husks, weeds, grasses, wood, or other waste products. This option is advantageously implemented by a gasifier that generates fuel gas from biomass and supplies this fuel gas to the burner of a free-piston Stirling engine, although other external combustion engines can be used. The mechanical power output of such an external combustion engine can be connected to drive a mechanical load or an electrical alternator to supply power to electrical appliances.
Gasification is the process of producing energy by converting organic or fossil fuel based carbonaceous materials, through pyrolysis at high temperatures, into carbon monoxide, hydrogen, and carbon dioxide and has been practiced for two centuries. During both world wars, especially World War II, there was a need for fuel produced by gasification due to the shortage of petroleum. The downdraft gasifier was first popularized during World War II due to increased prices and demands for fuel, and the robust ability of the downdraft gasifier to supply fuel to an engine.
A prior art downdraft gasifier typically has a tubular, vertically oriented fire tube that surrounds a gasifier working chamber. Biomass is fed into the top of the working chamber and is processed as the biomass progresses downward through a series of processing zones. Resulting ash and biochar exit from the bottom of the fire tube into an ash collection receptacle. The processing zones are most commonly referred to as, in downward progression, the drying zone, the pyrolysis zone, the combustion zone and the reduction zone from which the ash and biochar is dropped into a collection receptacle. Gases that result from pyrolysis flow downward within the working chamber. Ambient air to support combustion is introduced into the combustion zone via ports or pipes through the fire tube sidewall at the level of the combustion zone.
Although the above terminology is adopted for describing the invention, there can be some variation from that terminology. Pyrolysis is sometimes defined as the thermal decomposition of the volatile components of organic material due to an indirect source of heat at temperatures of 200° C. to 760° C. in the absence of oxygen. Gasification has been defined as a similar process which occurs with limited oxygen at temperatures of 480° C. to 1650° C. In view of the facts that these temperature ranges overlap and that air moves downward through the biomass during operation of the gasifier it is likely that both pyrolysis and gasification occur. Because the terminology choice does not have any effect upon the invention, the term “pyrolysis” is used for consistency.
In order to maintain the proper downward gas flow direction through the working chamber, prior art downdraft gasifiers have a removable lid at the top end of the fire tube to prevent upward gas flow by convection through the working chamber and to limit air flow into the working chamber from the top while also permitting the lid to be temporarily removed at times for adding biomass into the working chamber. The fuel gas produced by pyrolysis passes from the bottom of the reduction zone through an outlet tube or other passageway that typically turns vertically upward and extends above the level of the ambient air intake so that proper gas flow direction is maintained by convection through the gasifier working chamber and the outlet tube.
Since all tars must pass through a hot bed of char in the downdraft configuration, tar levels in the produced fuel gas are lower than in an updraft gasifier. However, despite that advantage, these gasifiers were commonly operated for only short periods of time because long term operation for much more than two hours was difficult due to the amount of tar and other pollutants in the fuel gas which would accumulate and disrupt proper operation of the engine.
Presently, long term, continuous operation remains a problem because gasifier operation must be halted periodically due to the formation of tar. The best gasifiers currently available can be run for only approximately 20 hours. Another problem with current downdraft gasifiers is that they require a closure or lid at the top of the fire tube in order to limit the incoming air because the air flow through the fire tube is downward which is opposite the direction of air flow through the fire tube that would be expected from simple convection. Therefore, some means must be provided to maintain the downward gas flow through the gasifier. One means shown in the prior art is an outlet passageway that extends sufficiently above the combustion zone in the fire tube so that it functions like a chimney to draw gas downward through the fire tube and then upward through the outlet passageway. However, such an outlet passageway still requires a closure at the top of the fire tube that must be removed from time to time to refill biomass into the fire tube.
A wide diversity of biomass materials are available at different times and places for fueling the burner of an external combustion engine. Sometimes biomass is available in granular or pellet form. It would be desirable to provide a burner that allows such biomass particles to be fed directly into the burner and burns them efficiently.
It is, therefore, an object and purpose of the invention to provide gasifier improvements that allow long term continuous gasifier operation for several days.
A further object and purpose of the invention is to provide a power source that has improved combustion efficiency of fuel gas produced by a gasifier.
Yet another object and purpose of the invention is to improve the ease and convenience of operating a downdraft gasifier by eliminating the need for a closure at the top of the fire tube. Without the top closure biomass can be conveniently manually filled into the fire tube without removing a hot top closure and additionally an automatic biomass feeder can be installed thereby extending the time between periodic inspection and operation by an attendant.
Yet another object of the invention is to provide a burner that is capable of burning any of a wide variety of fuel including other combustible gaseous fuels such as propane, natural gas, or landfill gas, liquid fuels and also allows solid biomass particles to be fed directly into the burner without the need for converting them to fuel gas.